An internal combustion engine may include one or more turbochargers for compressing a fluid which is supplied to one or more combustion chambers within corresponding combustion cylinders. Each turbocharger typically includes a turbine driven by exhaust gases of the engine and a compressor which is driven by the turbine. The compressor receives the fluid to be compressed and supplies the fluid to the combustion chambers. The fluid which is compressed by the compressor may be in the form of combustion air or a fuel and air mixture.
A turbocharger may also include a two stage compressor with two separate compressor wheels which are carried and driven by a common shaft. U.S. Pat. No. 4,344,289 (Curiel et al.) discloses a supercharger with a two-stage compressor having two compressor wheels which are disposed in a back-to-back orientation relative to each other and carried by a common shaft. The hub portions of the two compressor wheels are configured differently relative to each other, but the overall diameter of the compressor wheels at the outside diameter of the blades is the same. The two compressor wheels and the common shaft appear to be monolithically formed with each other. Since the compressor wheels have the same diameter, the stress capacity of the compressor wheels is limited by the common material from which they are constructed. Moreover, the rotational inertia of the entire compressor assembly can only be altered by changing the geometry of the compressor wheels since they are constructed from a common material.
An exhaust gas recirculation (EGR) system is used for controlling the generation of undesirable pollutant gases and particulate matter in the operation of internal combustion engines. Such systems have proven particularly useful in internal combustion engines used in motor vehicles such as passenger cars, light duty trucks, and other on-road motor equipment. EGR systems primarily recirculate the exhaust gas by-products into the intake air supply of the internal combustion engine. The exhaust gas which is reintroduced to the engine cylinder reduces the concentration of oxygen therein and increases the specific heat of the mixture, which in turn lowers the maximum combustion temperature within the cylinder, decreasing the formation of nitrous oxides (NOx). Furthermore, the exhaust gases typically contain unburned hydrocarbons which are burned on reintroduction into the engine cylinder, which further reduces the emission of exhaust gas by-products which would be emitted as undesirable pollutants from the internal combustion engine.
It is known to recirculate the exhaust gas within a turbocharger in an internal combustion engine. For example, U.S. Pat. No. 4,250,711 (Zehnder) discloses a supercharger for an internal combustion engine with exhaust gas recirculation. The supercharger includes a single turbine wheel and a single compressor wheel. The turbine is in the form of an axial turbine which is driven by exhaust gas from an exhaust manifold of the internal combustion engine. A portion of the exhaust gas may be recirculated from the turbine side to the compressor side of the supercharger. More particularly, the exhaust gas is recirculated to the discharge side of the single stage compressor either immediately prior to the diffuser section or within the volute section. A single stage compressor as shown in Zehnder, 711 has limited compression capabilities, primarily dependent upon the configuration and rotational speed of the compressor wheel.
The present invention is directed to overcoming one or more of the problems as set forth above.